The present application relates to a batter, charger for a charging battery pack of secondary batteries.
Battery chargers for charging secondary, batteries using commercial power sources have been known. The present inventors have already proposed a battery charger described in Japanese Patent No. 3430264 (Japanese Unexamined Patent Application Publication (KOKAI) No. H06-14473: Patent Document 1).
FIG. 1 shows a configuration similar to that shown in the Patent Document 1. Commercial alternating current (referred to as “AC” for convenience' sake, hereinafter) power source is converted into a DC power source by an input filter 1 and a rectifier circuit 2. A switching power source includes a pulse width modulation control circuit 3, a transistor Q1, and a transformer T1. The transistor Q1 as a switching element performs switching operation, for example, at 100 kHz, by output pulses from the pulse width modulation control circuit 3. Rectified output of a diode D1 and a capacitor C1, connected to a tertiary winding N3 of the transformer T1, is supplied as a power source of the pulse width modulation control circuit 3.
The transistor Q1 regulates current flowing through a primary winding N1, and correspondent electric power is induced on a secondary winding N2 and the tertiary winding N3. A voltage induced on the secondary winding N2 is rectified by a diode D2 and a capacitor C2 to obtain a rectified output Vo. The rectified output Vo is extracted through a switching unit 4 composed of an FET F1, an FET F2, and a transistor Tr1 and the like, between output terminals 5a [positive(+)side] and 5b [negative(−)side].
A secondary battery BAT such as a lithium ion secondary battery, is connected between the output terminals 5a and 5b. The secondary, battery BAT is connected in attachable/detachable manner to/from the battery charger. The battery charger includes a switch SW for detecting attachment/detachment of the secondary battery BAT. Upon attachment of the secondary battery BAT, the switch SW turns on, and a detection signal Batt at L (which means LOW level, the same applies hereinafter), indicating that the secondary battery BAT is attached, is supplied to a controller 11 composed of a microcomputer.
The rectified output Vo is divided by a resistor R7 and a resistor R8 to input to the negative(−)terminal of an operation amplifier AMP1. On the other hand, the positive(+)terminal of the operation amplifier AMP1 is supplied with a reference voltage REF1. The output voltage Vo is compared with the reference voltage REF1, and an error signal indicating difference from the reference voltage is supplied to a photocoupler PH1 via a diode D3.
The error signal transmitted from the secondary side to the primary side of the photocoupler PH1 is supplied to the pulse width modulation control circuit 3. The pulse width modulation control circuit 3 controls an ON period of output pulses from the transistor Q1, so as to control electric power to be supplied to the secondary side, whereby an output voltage set by the reference voltage on the secondary side is extracted.
An output (charge) current Io is detected by a resistor R2. The load-side (output-side) terminal of the resistor R2 is connected to the negative terminal of an operation amplifier AMP2 via a resistor R5. A voltage obtained by dividing the reference voltage REF1 by resistors R4 and R6 is supplied to the positive terminal of the operation amplifier AMP2, to thereby raise voltage level at the positive terminal of the operation amplifier AMP2.
Flow of output current Io induces voltage drop over the resistor R2 ascribable to the output current Io. As a consequence, a voltage divided be the resistors R4 and R6 decreases. Increase in the output current Io causes further voltage drop at the positive terminal of the operation amplifier AMP2. When the potential at the positive terminal of the operation amplifier AMP2 falls down to the potential at the negative terminal or therebelow, the output signal from the operation amplifier AMP2 shifts from H (which means HIGH level, the same applies hereinafter) to L.
The output signal from the operation amplifier AMP2 is supplied to the pulse width modulation control circuit 3 via a diode D4 and a photocoupler PH1, so that the power control is performed similarly to voltage control. More specifically, voltage drop occurs at the positive terminal of the operation amplifier AMP2 depending on the amount of current flowing through the resistor R2, the potential of the positive terminal is compared with that of the negative terminal, and the amount of output current is controlled to keep voltage generated at the resistor R2 constant. The output current is regulated at a constant level in this way.
A predetermined voltage stabilized from an output voltage Vo by the regulator 12 is supplied to the controller 11 as a source voltage. An LED (light emitting diode) 13 as a display element, indicating the state of charging operation, is connected to the controller 11.
The switching unit 4 is operated by drive pulse signals DR1, DR2, and DR3 outputted from the controller 11. When the controller 11 detects the attachment of the secondary battery BAT by receiving the detection signal Batt, charging operation starts to perform a predetermined charging operation under monitoring of battery voltage Vbatt.
The above-described battery charger charges the secondary battery BAT based on a CC-CV (constant current-constant voltage) charging system, which is a combined system of constant-current charging and constant-voltage charging. FIG. 2 shows output characteristics of the above-described battery charger. The abscissa represents charging current, and the ordinate represents charging voltage. The battery charger first operates in the constant-current control mode, for example, at 1.0 A, and then operates in the constant-voltage control mode, for example, at 4.2 V. In the initial charging mode in the early stage of charging, the charging at initial charging current If is proceeded. When the voltage reaches a rapid switching voltage, for example, at 2.7 V, the charging mode switches to a rapid charging mode.
FIG. 3 shows time-dependent changes(charging curve) in the charging voltage and charging current during charging. For example, the constant-current control proceeds in a region where the battery voltage is as high as the constant-voltage control voltage (4.2 V, for example) or below, whereby the constant-current charging is performed under a constant current (1.0 A, for example). When the battery voltage (internal electromotive force) elevates to reach 4.2 V as a result of charging, the battery charger switches the operation into those under the constant-voltage control, whereby the charging current gradually decreases. When the charging current is detected to reach the end-of-charging detection value Is, the end-of-charging is detected. From this point in time, a float timer activates, and the battery is charged until the time-out to terminate the charging of the battery. The charging adopts the floating timer, because the capacity may slightly be increased even after the point in time when the end-of-charging is detected.
In the configuration shown in FIG. 1, during the constant-current charging, the output of the operation amplifier AMP2 is supplied to the photocoupler PH1 via the diode D4, and the power source is regulated to give constant output current. During the constant-voltage charging, the output of the operation amplifier AMP1 is supplied to the photocoupler PH1 via the diode D3, and the power source is regulated by the output voltage of the operation amplifier AMP1 so as to give constant output voltage Vo. In the configuration shown in FIG. 1, one end of the load-side of the current detecting resistor R2 is connected to the negative terminal of the comparator 6, the other end of the load-side is connected to the negative side of a reference voltage REF2, and the positive side of the reference voltage REF2 is connected to the positive terminal of the comparator 6. The charging current is converted into voltage by the resistor R2, and the voltage is compared with the reference voltage REF2. When the charging current decreases, the reference voltage at the positive terminal of the comparator 6 becomes larger than the detection voltage at the negative terminal thereof, whereby an output Cs of the comparator 6 inverts. The output Cs of the comparator 6 is supplied to the controller 11, and the controller 11 detects the end of charging.
The known battery charger which detects the end of charging in this way needs to provide the reference voltage REF2 for detecting end of charging, and needs to use a precision-offset comparator having a small offset voltage as the comparator 6 for detecting end of charging, which is an expensive component.
A similar battery charger is described also in Japanese Unexamined Patent Application Publication (KOKAI) No. 2007-20299 (Patent Document 2).
The Patent Document 2 proposes a method of improving sensitivity of current detection. The method is performed by switching the resistance value for detecting charging current to a larger value when the charging current decreases to fall below a set value. In this case, a change-over switch for changing the resistance value is necessary. In any attempt of using, for example, an FET element for the switch, it is necessary to select an FET considerably small in the resistance value, which leads to require an expensive FET.